Share 'Giveaways--Tacky if they run on more than one blog?'

I'm thinking about sending several mom blogs requests to do a giveaway for us. But I'm wondering, what is the etiquette about running the giveaway on more than one blog? I know, as a mom blogger, it's disheartening to have someone else tweeting/facebooking/etc about the same giveaway on their own blog, but what if the giveaways run at different times? Would really appreciate your thoughts.